TO 



MECHANICAL PUMP 



Fig. 5. Schematic representation of apparatus used in shadowing for electron micro- 

 scopy. The filament is normally charged with a small amount of a heavy metal such as 

 uranium or platinum. The "shadowing angle" is defined as the reciprocal of the tangent of 

 the angle a. (From The Anatomical Record 96, 27, 1946) 



Fig. 6. (a) Particles of tobacco mosaic virus, not shadowed; (b) shadowed with uranium. 

 Both at X 150,000. Although the shadowed particle does not appear smooth on its 

 surface, there is no evidence of periodic structure. 



